1. Field of the Invention
The present invention relates to an optical apparatus having a function of correcting image shakes caused by changes of relative angles of an object to be photographed and the optical apparatus.
2. Description of Related Art
Optical apparatuses of the kind arranged to be capable of correcting image shakes have heretofore been developed in varied manners. FIG. 7 shows an example of such optical apparatuses. In the case of the optical apparatus shown in FIG. 7, the so-called variable angle prism 100 is arranged in front of an optical system composed of lens units 101 to 104 and a diaphragm 105 to correct the image shakes before a light flux P from an object comes to be incident on the optical system. Referring to FIG. 7, the lens units 101 to 104 and the diaphragm 105 are supported by a fixed tube 105. An image sensor 107 is arranged to convert into an electrical signal an optical image formed on a focal plane which is located in rear of the fixed tube 106.
In the above optical apparatus, however, the variable angle prism 100 is disposed in a place where the width of the light flux P passing through the optical system becomes widest. Therefore, the arrangement for having the variable angle prism 100 in that position is disadvantageous in respect of reduction in size of the optical apparatus.
In view of the above problem, some of known optical apparatuses have been developed to permit reduction in size. For example, an optical apparatus is arranged to have the variable angle prism disposed between two lens units within an optical system composed of a plurality of lens units. Another optical apparatus which is of the type called a lens shift type is arranged to correct the image shakes by moving some of a plurality of lens units in a direction perpendicular to an optical axis. Some other optical apparatus has been developed to have an electronic image-shake correcting function called an electronic image stabilizing device which corrects image shakes in the following manner. A CCD which has a larger area than an actually necessary area as an image sensor (thus requiring use of a large optical system having an image circle covering the whole surface of the CCD) is arranged to correct image shakes by varying the reading position thereof according to information on image shakes detected.
With the conventional optical apparatus arranged as described above, for example, in an optical system composed of lens units 111 to 114 having positive, negative, positive and positive refractive powers, respectively, and a diaphragm 115, as shown in FIG. 8, while an on-axial light flux “a” passes the center of the optical system, the upper and lower parts of an off-axial light flux “b” are blocked by the effective diameters, indicated by arrows A and B, of the lens units 111 and 114. Therefore, the thickness of the light flux becomes smaller accordingly as the incident angle of the light flux is larger (corresponding to the periphery of an image plane). As a result, an image formed by the light flux on an image forming plane 116 rapidly becomes darker in its peripheral part. The peripheral light quantity of the light flux is thus decreased by the so-called vignetting phenomenon.
In the case of the above-stated function of optically correcting image-shakes by deflecting a light flux within the optical system, if the optical apparatus is arranged to correct the image shakes in such a way as to have an object image not moving on the image forming plane at the time of a change of relative angles of the object and the optical apparatus, the vignetting degree of the light flux at each point of an object image on the image forming plane would vary to change the light quantity distribution of the object image, as the relative angles of the object and the optical apparatus have changed.
Such a state is explained with reference to FIG. 9. In FIG. 9, image forming positions are shown on the abscissa axis and the luminance of the image is shown on the ordinate axis. A one-dot-chain line shown in FIG. 9 represents the initial distribution of light quantity obtained before the relative angles of the object and the optical apparatus change. Full lines “a” and “b” shown in FIG. 9 represent light quantity distributions obtained with image shakes corrected when the relative angles change, for example, alternately to the right and to the left. When image shakes which actually take place continuously are corrected in this manner, although the object image is corrected to be not moving on an image plane, the image shake correction results in variations of luminance of the picture taking place in synchronism with the image shakes. The luminance variations become salient particularly in the peripheral part of the image plane. As a result, the quality of an image thus obtained degrades to an unacceptable degree.
Further, the electronic image shake correcting function also has a problem similar to that of the optical image shake correcting function. In this case, the light quantity distribution on the image forming plane does not change, since the relation between the image forming plane 116 and the optical system which is composed of the lens units 111 to 114 and the diaphragm 115 as shown in FIG. 8 is unchanging. However, when the object image is caused to move by the change of relative angles of the object and the optical apparatus, the reading position also changes following the movement of the object image, for example, as indicated by reading positions I and II in FIG. 10. The change of the reading position then brings about the same phenomenon as in the case of the optical image shake correcting function.